Nitrogen and phosphorus contained in soil water are causative agents for eutrophication problems in rivers, oceans and water storage ponds, and their efficient removal by a sewage treatment process is desired.
Phosphorus resources are substances whose exhaustion in the 21st century is expected. Japan depends on imports for most of its phosphorus supply. Nowadays, there is a demand for a method for recovering phosphorus from organic wastes and waste water with high efficiency.
Various methods, such as a biological removal process, a coagulation-sedimentation process, a crystallization process, and an adsorption process, have so far been developed as methods for removing phosphorus from soil water containing phosphorus. These different treatment methods each have advantages and disadvantages. Of the methods, the crystallization process is advantageous in that it is basically free from the occurrence of sludge, recycling of the removed phosphorus is easy to perform, and phosphorus can be removed (recovered) in a stable state.
(Patent Document 1) describes a method for recovering phosphorus as magnesium ammonium phosphate (which may be hereinafter referred to as “MAP”), from waste water containing high concentrations of phosphorus and ammonia nitrogen. According to this MAP process, ammonium ions, phosphate ions, magnesium ions, and hydroxyl groups in liquids react in a manner as indicated by the formula (1) to form MAP. The resulting MAP can be recycled as a response-delayed fertilizer (magnesium ammonium phosphate-based).
[Chemical Formula 1]Mg2++NH4++HPO42−+OH−+6H2O→MgNH4PO4.6H2O (MAP)+H2O  (1)
When organic wastes such as sewage sludge, raw garbage, stock farm wastes, malt and used tea leaves, are anaerobically digested, they are finally decomposed into carbon dioxide and methane via a solubilization step for solid matter, an organic acid formation step, and a methane formation step. These organic wastes contain phosphorus and nitrogen, which are eluted into the liquids during the anaerobic digestion step. According to prior arts, MAP was formed from filtrates obtained by dehydration of these sludges, and the MAP was collected. The formation of MAP required phosphorus, ammonia nitrogen, magnesium, and an alkali in the liquids as described above and, if the filtrates lacked any of these substances, it was added.
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-326089